Hi...new to all this but just had to respond.A friend has a 2 year old lab with the same problem. They went down the same route as yourselves but everything came back. He has just had allergy checks 3 weeks ago and they have found he is allergic to meadow grass, dust mites and 2 other things which I can't remember. They now walk him in a different area to the fields he was used to and that has helped but have yet to make him an all over suit for the house! May be worth an allergy test.

Hope you get them sorted out though.